1. Field of the Invention
This invention pertains to the in-service measurement of the bit error rate (BER) of a data stream in a digital communications circuit.
2. Description of the Related Art
A method commonly used to measure errors in a digital communications circuit is described in U.S. Pat. Nos. 3,315,228, 3,596,245, 3,725,860, 3,760,354, 3,824,548, 3,914,740, and 4,428,076. This method uses a code or pattern generator which injects a known test pattern into a digital communications circuit. At the other end of the circuit the received pattern is compared with a synchronized, locally generated version of the known test pattern (see FIG. 1). This comparison yields a count of bit errors. The BER is derived by dividing the number of bit errors by the total number of bits observed.
U.S. Pat. No. 4,234,954 describes a method in which signal level violations are detected, accumulated, and used to estimate the communications circuit BER.
U.S. Pat. No. 4,397,020 describes a method where cyclic redundancy codes (CRCs) are used to detect errors in a communications circuit. The CRC is calculated at the transmit end on a block of 4632 bits and then embedded into the data. At the receive end, the CRC is recalculated on the same block of data and compared to the embedded CRC; any differences indicate the presence of errors.
The prior art suffers from one of two major disadvantages: it either requires the communications circuit to be taken out of service (hence, it is termed an out-of-service test), or it requries the data to be altered (hence, it is an intrusive test).
U.S. Pat. Nos. 4,710,924 and 4,713,810 describe a technique for storing BER results in remote monitoring points along a digital communications circuit for subsequent retrieval. This technique provides the means to isolate faulty sections of the communications circuit. However, this technique does not address the method of measuring the BER.
U.S. Pat. No. 3,916,379 and commonly owned U.S. Pat. No. 4,736,377 describe a method of non-intrusive testing that employs a secondary digital communications channel. Synchronization information and error codes or cyclic redundancy codes (CRCs) are sent from one end of the digital communications circuit to the other end via this secondary channel. These codes, which in the prior art are computed over adjacent, uniformly sized blocks composed of contiguous bits of data, are compared to codes identically computed on the received digital information; each miscompare is accumulated as a block error. In U.S. Pat. No. 3,916,379, the BER is then estimated from the number of block errors. However, this estimate rapidly loses accuracy as the block error rate (BLER) approaches unity (i.e., at high BERs and/or large block sizes). Furthermore, if the block size is reduced to accommodate high BERs, then the secondary channel bandwidth required to transmit the error codes increases proportionally. This is a disadvantage since secondary channel bandwidth is usually limited by cost and availability.